Water Activity in Page 1

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Water Activity (aw) in Foods

CONTENTS SUMMARY

1. Introduction

2. The Structure of Water

3. Water In Foods

3.1 Water and 3.2 Water Activity (aw) 3.3 Effects of Reduced aw on Food Safety Water is the most abundant 3.4 To Reduce aw constituent of food and in terms of food safety the most significant. Its pres- 4. Measure of Water Content and aw ence, quantity and nature 4.1 Relationship of Water Activity and Water determines many chemical 4.2 Analysis of Water Content and biochemical processes important for the control of 4.3 Analysis of Water Activity product safety and quality. 4.4 Water Activity as a CCP A better understanding of

water activity aw can assist in developing robust and scientifically supported food safety plans.

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1. Introduction At standard temperature and pressure water is a liquid. It is also tasteless and Water is the most abundant constituent odorless. The molecule itself is non-linear of food and in terms of food safety the and polar with an electrical dipole most significant. Its presence, quantity movement. It is a good polar solvent and and nature determines many chemical and is typically referred to as the universal biochemical processes important for the solvent. control of product safety and quality. Hydrophilic (water-loving) substances In many HACCP plans, water is frequently dissolve readily while hydrophobic (water- referred to as an intrinsic parameter fearing) substances are immiscible. At required in product safety and while most standard temperature and pressure the of us understand its importance we often boiling point of water is 100 °C (212 °F). fail to understand the reasons why. A better understanding of water, and in The density of liquid water is 1,000 kg/m3 particular water activity (aw), can assist (62.43 lb/cu ft) at 4 °C. Ice has a density of in developing robust and scientifically 917 kg/m3 (57.25 lb/cu ft). supported food safety plans. 3. Water In Foods

2. The Structure of Water Water in food is an important parameter in the field of and safety. Due The chemical structure of water is H2O. It to its unique role in various chemical and does not exist in an absolutely pure form biochemical reactions in a food matrix an in nature due to its chemical properties. understanding of water is crucial. One molecule of water is composed of two 3.1 Water and Food Safety hydrogen atoms covalently bonded to a single oxygen atom. It commonly occurs One of the oldest forms of preserving foods in all three states, i.e. liquid, solid and gas. is drying or . When first applied there was little understanding of why removing moisture from food had the effect of extending its .

The table on the following page shows the typical water content of certain food product and categories:

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Food % Water Food % Water Lettuce, tomatoes 95 Cabbage, broccoli 92 Carrots, potatoes 90 Citrus fruit 87 Apples, cherries 85 Raw poultry 72 Raw lean meat 60 Cheese 37 White bread 35 Salami 30 Preserves 28 20 Dried fruit 18 Butter 18 Wheat flour 12 Dried pasta 12 powder 4 Beer 90 Fruit juices 87 Milk 87

Over time, an understanding emerged and To understand this more fully, we must today we have a solid scientific recognise that much of the water in food understanding of how water impacts on is bound water, i.e. bound to ions as water product quality and safety. We now know of hydration, or bound to surfaces of large that the water content is less important molecule or cell structures. This water is than the specific behaviour of the water or not free to support microbial growth, or Water Activity. Water activity is related to to participate in or support chemical or water content in a non-linear relationship enzyme reactions and spoilage processes. which is represented using as a moisture The total amount of bound water in a food sorption isotherm curve. has no relation to food stability.

3.2 Water Activity (aw) The free or available water in a food supports microbial growth, and This is the most important parameter participates in and supports chemical and of water in terms of food safety. Water enzymatic reactions and spoilage processes. activity or aw is the partial It is this amount of free water which is of water in a substance divided by the called water activity, aw, and it is more standard state partial vapor pressure of important for food stability, chemical and water. microbial, than total water content.

In the field of food science, the standard The factors that reduce the mobility of state is most often defined as the partial water in a food also reduce its tendency vapor pressure of pure water at the to evaporate and its vapour pressure and same temperature. Using this particular this provides a means of defining and definition, pure distilled water has a water measuring it. activity of exactly one.

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Therefore, we can generally define Water Another chemical reaction effect is

Activity (aw) as an indication of the amount Maillard Browning which is maximum at of Free Water in a food. 0.6-0.7. Most enzymes are inactivated at <0.85. At less than 0.75 bacterial growth

Specifically Water Activity (aw) is: is inhibited but some yeast and moulds may grow. At less than 0.6 all growth is = Actual Equilibrium v.p. of air-space inhibited. In terms of nutrients, reduced over the food aw reduces losses of Vitamins C, E, B1. The following table indicates the aw of Equilibrium v.p. of pure water at certain foods. same temperature

Food Typical aw Alternatively we can define it using Fresh meat 0.98 Relative (RH): Cheese 0.97 Preserves 0.88 = Actual v.p. water vapour in air x 100 Salami 0.83 Dried fruit 0.76 Equilibrium v.p. of Pure Water at Honey 0.75 Same Temperature Pasta 0.50 Water Activity Table: Water activity values of foods = Relative Humidity of Airspace In general, bacteria require higher Over the Food / 100 values of aw for growth than fungi, with The ideal solution may be written as : gram-negative bacteria having higher requirements than gram-positive. Aw = P = %ERH Organism group a Po 100 w Most spoilage bacteria 0.90 Most spoilage yeasts 0.88 Most spoilage moulds 0.80 3.3 Effects of Reduced wa on Food Safety Halophilic bacteria 0.75 Xerophilic moulds 0.61 The rate of certain chemical and Osmophilic yeasts 0.61 biochemical processes are effected by the amount of available water. One of the effects of reducing the aw in a food product Table (above): Minimum aw values for is to reduce the rate of these reactions. The growth of groups in food exception is the oxidation of fats where Table: Minimum a values for growth of the rate decreases down to 0.4 to 0.5, then w specific in food increases.

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Organism aw Organism aw , type E 0.97 Candida scottii 0.92 Pseudomonas spp. 0.97 Trichosporon pullulans 0.91 Acinetobacter spp. 0.96 Candida zaylanoides 0.90 Escherichia coli 0.96 0.86 Enterobacter aerogenes 0.95 Alternaria citri 0.84 0.95 Penicillium patulum 0.81 Clostridium botulinum, types A and B 0.94 Aspergillus conicus 0.70 Candida utilis 0.94 Aspergillus echinulatus 0.64 Vibro parahaemolyticus 0.94 Zygosaccharomyces rouxii 0.62 Botrytis stolonifer 0.93 Rhizopus stolonifer 0.93 Mucor spinosus 0.93

For example, where a product is sold by 3.4 To Reduce aw weight, controlling its water content may Reducing the aw of a food product may be be important to meet legal and commercial achieved through a number of methods. requirements. Water activity is more The most obvious is by partial removal of significant for qualitative considerations water in the food product using a variety such as product stability, shelf life (e.g. of unit operations or processes. The microbiological & enzymatic stability, concentration of water can also be reduced aroma retention), handling characteristics, by the addition of other substances physical properties and chemical stability. including and . Many of these methods have been used long before the Water activity and water content can concept of water activity was understood. be related by a graph called a Sorption Isotherm (see diagram), so if the user has the ability to measure both 4. Measure of Water Content parameters, the relationship can be

and aw defined and each parameter derived from the other (interpolation). 4.1 Relationship between Water Activity and Water In practice, the sorption isotherm may be impractical to use because, not only does There is often confusion between water the relationship between Aw and moisture activity and water content measurements. content change with temperature of In many sectors, water content is used to measurement, but also any variations in control the amount of water present in a the material composition has a modifying product for quantitative reasons. effect.

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The company should therefore decide that show erratic results when heated or which parameter of measurement is best subjected to vacuum, e.g., low moisture suited to their products and processes. For foods such as dried fruits and vegetables, quality control purposes, moisture content candies, chocolates, roasted coffee, oils and limits are easily converted to water activity fats or any low moisture foods rich in sugar limits by very simple comparative tests. or protein. The method is based on the Water activity measurement offers a non- fundamental reaction described by Bunsen destructive, easy-to-use measurement in a in 1853 involving the reduction of Iodine wide range of convenient configurations for by SO2 in the presence of water. both laboratory and on-site use. 2H2O + SO2 + I2 → C5H2SO4 + 2HI Picture: Sorption Isotherm This was modified to include methanol and pyridine in a four component system to dissolve the Iodine and SO2. For each mole of water, 1 mole of iodine, 1 mole of SO2, 3 moles of pyridine, and 1 mole of methanol are used.

In the titration, if a sample containing water is titrated with a KF reagent, iodine 4.2 Analysis of Water Content is consumed while there is water in the system. When no water remains then free Gravimetric Method iodine appears and can be detected using two platinum electrodes immersed in the Water content may be measured in food solution being titrated. using a number of methods. The most basic of these are gravimetric methods. This As water is titrated, the voltage of the involves drying a known quantity of the electrodes approaches virtually zero. And food product in an oven until all moisture where there is an excess of iodine (all the has been evaporated. By measuring the dry water has been consumed) a current can be matter content remaining, the water measured. content can be determined. A vacuum oven can be used for heat sensitive foods. 4.3 Analysis of Water Activity

Karl Fisher Titration A number of methods can be employed to measure the water activity including a A more sophisticated method for the resistive electrolytic, a capacitance or a dew analysis of water content is the Karl Fisher point . Titration. The Karl Fisher titration is particularly adaptable to food products

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Resistive electrolytic Capacitance hygrometers

Resistive electrolytic hygrometers Capacitance hygrometers consist of two use a sensing element in the form of a charged plates separated by a polymer liquid electrolyte held between two small membrane . As the membrane glass rods by capillary force. The adsorbs water, its ability to hold a charge electrolyte changes resistance if it absorbs increases and the capacitance is measured. or loses water vapor. This value is roughly proportional to The resistance is directly proportional to the water activity as determined by a relative air humidity, and also to the sensor-specific calibration. Capacitance water activity of the sample (once vapor– hygrometers are not affected by most liquid equilibrium is established). This volatile chemicals and can be much smaller relationship can be checked by either a than other alternative sensors. verification or calibration using salt- water mixtures, which provide a well- They do not require cleaning, but are less defined and reproducible air humidity in accurate than hygrometers the measurement chamber. (+/- 0.015 aw). They should have regular calibration checks and can be affected by The sensor does not have any physically residual water in the polymer membrane given hysteresis as it is known from (hysteresis). capacitance hygrometers and sensors, and does not require regular cleaning as Dew point hygrometers its surface is not the effectively sensing The temperature at which dew forms on a element. clean surface is directly related to the vapor Volatiles, in principle, influence the pressure of the air. Dew point hygrometers measurement performance—especially work by placing a mirror over a closed those that dissociate in the electrolyte sample chamber. and thereby change its resistance. Such The mirror is cooled until the dew point influences can easily be avoided by using temperature is measured by means of an chemical protection filters that absorb the optical sensor. This temperature is then volatile compound before arriving at the used to find the relative humidity of the sensor. chamber using charts.

This method is theoretically the most accurate (+/- 0.003 aw) and often the fastest. The sensor requires cleaning if debris accumulates on the mirror.

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4.4 Water Activity as a CCP In some cases it may be a legal requirement e.g. FDA Regulations. Water activity is frequently used as a However, it is important to know that for Hazard Analysis lowering of the water activity of a food and Critical Control Points (HACCP) product is not in itself a lethal kill step programs. Samples of the food product can since some pathogens can survive. be tested to ensure water activity values are within a specified range for food quality Where it is used as a CCP or a specific and safety. control point it should be based on full validation of the product and process with Measurements can be made in as little as clear limits. five minutes, and are made regularly in most major food production facilities.

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